The present invention is directed to a hydropneumatic axle suspension for vehicles having greatly varying axle loads, in particular for front axles on tractors having hydraulic suspension cylinders.
In hydropneumatic axle suspensions, it is customary for a level-control system to be used. As the axle load increases, the ride height of the vehicle is reduced, and a level-control valve is adjusted to allow a hydraulic fluid to be fed via a pressure line into the cylinder chamber of the hydropneumatic actuators until the original level is reached again. As the axle load decreases, the ride height of the vehicle is increased, and the level-control valve connects the cylinder chambers of the hydropneumatic actuators to a discharge line, until the original level is restored.
Besides the level control, vehicles having a high load ratio are equipped with a pressure control for the annular spaces of the suspension cylinders, which, also by varying the pressure level, enables the suspension performance of the vehicles to be adapted to different load conditions. At the present time, the pressure level prevailing in the annular spaces of the suspension cylinders is regulated only as a function of the existing axle load. A suspension system made up of a level control and a pressure control is described in the German Patent Application DE 41 20 758 A1.
From the German Patent DE 42 42 448 C1, an improved hydropneumatic suspension device has become known, where a load-sensing pump is used, and whose level-control device is provided with a valve device which, in response to a static load change, briefly regulates the pressure level of all control lines and supply lines up or down and, in otherwise assumed neutral positions, relieves the lines of pressure, the piston chambers and annular spaces being hermetically sealed off via non-return valves capable of being unblocked.
The non-prepublished German Patent Application DE 101 07 631 discusses a hydropneumatic axle suspension that permits an especially rapid pressure adaptation and, moreover, provides improved driving comfort for the vehicle driver. In the context of this suspension, it is possible to vary the pressure prevailing in the suspension-cylinder annular spaces to yield a different stiffness of the spring characteristic in different load ranges. In the low load range, the pressure in the annular spaces of the suspension cylinders is preferably raised; the result is a spring characteristic having an increased stiffness.
A further improvement in the suspension performance is achieved by an axle suspension as described in the German Patent Application DE 101 07 644 A1, which is likewise not prepublished. In this axle suspension, the pressure-regulating valve used for regulating pressure is designed as a proportionally regulated valve that is controlled by a control current from an electrical control unit. This unit electronically processes both the measured signals from a pressure sensor, which is linked to the piston chambers of the suspension cylinders, as well as functionally dependent signals. In this way, a spring characteristic is achieved that has a constructively predefined curve shape that is dependent upon the load and operating state. The axle-spring rate is continuously adapted, automatically, to the suspended axle load and the working state for every control action.